CouchDBDao:
 * Removed obsolete method currentTimestamp()
 * Explicit IOException catch that indicates communication error

UserService:
 * Log if store file could not be created

All relevant information about users (e.g. session mebership) are
accessible via UserService.
The method IUserService::getCurrentUser provides information about
the actual logged in user. If no user is logged in, this method will
throw a UnauthorizedException which will end up in 401 HTTP error.

* Make guest login working with new UserService
* Reuse existing guest username
* Adapted tests

switch to oauth2 authentication which allows facebook, twitter, google
etc authentication